The origin of continental crust : insights from the TTG rocks in the Trans-North China Orogen and eastern block of the North China Craton

Abstract

Earth’s early continental crust is represented by tonalite–trondhjemite–granodiorite (TTG) rock suite. Both horizontal (e.g., magmatic arc) and vertical (e.g., mantle plume, sagduction, delamination, etc.) tectonic models were proposed previously to interpret the origins of continental crust. However, controversies exist on (mafic) rocks as precursor of the felsic crust and the formation-mechanism of the TTG rocks. In this study, the North China Craton (NCC), one of the oldest continental blocks of the Earth (~3.8 Ga), is chosen to study the origin of the TTGs. The NCC is subdivided into the Eastern Block (EB) and the Western Block (WB) and the intervening Trans-North China Orogen (TNCO). While the WB is largely covered by a desert, the TNCO and EB contain large amount of TTGs that may provide crucial information on the origin of early continental crust. From the TNCO, we collected the ~2.5 Ga TTG from the Tianzhen and Xinpingbu areas of the Huai’an Complex and conducted the whole-rock geochemistry, zircon U-Pb dating and zircon hafnium isotopes analyses. Our results indicate a long-lived Neoarchean to Paleoproterozoic magmatic arc setting for the Huai’an gneiss complex of the TNCO and constrain that the subduction between the EB and WB started at ~2.5 Ga, followed by collisional amalgamation at ~1.85 Ga. From the EB, we studied the ~2.5 Ga TTGs from the Taipingzhai area, and analyzed their geochemical compositions, and water and trace element contents, oxygen and hafnium isotopic compositions for zircons separated from these rocks. Our study demonstrates that the TTG magma resulted from the hybridization of oceanic plateau and minor supracrustal materials. The oceanic plateau provided low-content but adequate water for water-fluxed melting, while the supracrustal materials led to slightly high-δ18O character of TTG. Combined two-stage mantle plume and sagduction model is applied on the generation of the Taipingzhai TTG to interpret the formation of voluminous TTG (high δ18O, low H2O) at ~2.5 Ga. Our study compared the Neoarchean TTGs from the TNCO and the EB of the NCC and reveal that the TTGs could be formed at contrasting tectonic regime, i.e., the TTGs from the TNCO were generated at horizontal tectonic setting, while the TTG from the EB were formed at vertical tectonic setting. We emphasized that before the starting of plate tectonics on the Earth, the formation of early continental crust was dominated by vertical tectonic regime.